## **Core Concept**
The Major Histocompatibility Complex (MHC) is a group of genes that play a critical role in the immune system. MHC genes are divided into three classes: I, II, and III. While MHC class I and II genes are primarily involved in antigen presentation to T-cells, MHC class III genes encode for various proteins with immune functions, distinct from antigen presentation.

## **Why the Correct Answer is Right**
MHC class III genes encode several proteins with immune functions, including components of the complement system, cytokines, and heat shock proteins. One of the well-known products of MHC class III genes is **C2 and C4** (components of the complement system), **factor B**, and **C2 and C4 deficiencies** are associated with increased susceptibility to infections and autoimmune diseases. These gene products play critical roles in the innate immune response.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Incorrect because while MHC class I molecules are crucial for presenting peptides from proteins inside the cell to cytotoxic T-cells, they are not encoded by MHC class III genes.
- **Option B:** Incorrect because MHC class II molecules are involved in presenting antigens to helper T-cells and are not products of MHC class III genes.
- **Option D:** Incorrect because although cytokines are involved in immune responses, the specific focus of MHC class III genes includes complement components and not primarily cytokines.

## **Clinical Pearl / High-Yield Fact**
A key clinical correlation is that deficiencies in the complement components encoded by MHC class III genes (like C2 and C4) can lead to **increased susceptibility to infections** and **autoimmune diseases**, such as Systemic Lupus Erythematosus (SLE).

## **Correct Answer:** . Complement components C2, C4, and factor B.